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Form of Corrosion (Galvanic Corrosion (Factors (Nature of Environment
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Form of Corrosion
Galvanic Corrosion
-This occur when two metals in contact with different electrochemical potential in presence of electrolyte
-This form a galvanic cell
-The driving force for corrosion is potential difference between two metals
Prevention
4. Add inhibitor into aqueous system, decreasing aggressive environment
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Factors
Nature of Environment
-when metal surround with solution contain salt/acid, corrosion accelerate as this increase the ionization
-when surround in dry place, harder to corrode as less electrolyte to ionize
Area Effect
-high ratio of cathode to anode leads to higher corrosion
-large cathode has more oxygen to react, leading to accelerate corrosion
Position of metal in Galvanic Series
- the further apart metals are in the series, the higher the chance of corrosion
-as there is higher potential difference between two metals, higher driving force
Intergranular Corrosion
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Prevention
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3.Solution Quenching
-Heat above 815 °C, dissolve chromium carbide and water quench it
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4. Altering Environment
- Increase pH and decrease oxidizing condition
Crevice Corrosion
localized attack which occurs when crevices, formed by lapped joints, or
areas of partial shielding, are exposed to corrosive environments.
Mechanism
Stage 2- Increase in acidity and chloride in crevice
-Dissolution of iron within crevice continue, increase + charge
-Chloride ion migrate from outside to crevice, forming iron(II) chloride, to balance charge
-Hydrolysis of iron(II) chloride, produce higher hydrogen and chloride ions, thus increasing acidity
Stage 3- Permanent Breakdown of passive film
-The presence of hydrogen and chloride accelerate the corrosion of passive film
-Hydrogen and chloride is highly aggresive
Stage 1-Depletion oxygen in crevice
-This leads to the formation of anode at crevice and cathode outside of surface
Stage 4-Propagation of Crevice Corrosion
-Crevice Corrosion continue to propagate
-It will only terminate when metal is perforated
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Pitting Corrosion
Extreme localized corrosion of metal surface of small areas that leads to formation of pits or holes on metal
Mechanism
2.Breakdown of passive film
-Due to corrosion potential > pitting corrosion
-Leads to formation of anodic sites while metal surface form cathodic site
3. Pitt Growth
-Autocatalytic process
-Localized production of positive ions in pit attract aggressive ions to migrate into pit
-Metal chloride hydrolyzed and deposit in pit where hydrogen ions diffuse inwards and replace the metal ions, leading pitt growth
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Conditions to Occur
2. Presence of halogen or aggressive ions
-Example of halogen ions :Chloride,Bromide, Iodide Ions
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